In stead of the formerly used vulcanized rubber as the main material, thermoplastic elastomers are now used frequently in automobile parts, electric household appliances, medical device components and miscellaneous goods because of their excellent productivity. Examples of such cases include olefin elastomers comprising ethylene-propylene copolymer and polypropylene, polyurethane elastomers, soft polyvinyl chloride and the like.
Each of these molding materials, however, has disadvantages in terms of scratch resistance, flexibility, moldability, economical efficiency and recycling ability. That is, olefin elastomer is relatively cheap and excellent in weather resistance and heat resistance. However, it is inferior to other materials in flexibility and scratch resistance. Polyurethane elastomer has excellent scratch resistance but has disadvantages in its poor flexibility and weather resistance. Moreover, it is expensive. Also, soft vinyl chloride is relatively cheap and excellent in weather resistance and scratch resistance but is disadvantageous in poor flexibility, low temperature characteristics and recycling ability.
In addition, there have been proposed some elastomer compositions with the use of hydrogenated derivatives of a vinyl aromatic compound-conjugated diene compound block copolymer (hereinafter referred to as a "Hydrogenated block copolymer"). For example, JP-A-50-14742, JP-A-52-65551 and JP-A-58-206644 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") each discloses a composition in which a hydrogenated block copolymer is blended with a rubber softening agent and an olefin resin. However, these compositions are also poor in scratch resistance similar to the case of olefin elastomers.
On the other hand, the air bag system is principally composed of a collision sensing device which detects collision of a high-speed movable body and an air bag apparatus. The latter air bag apparatus is composed of an air bag, a gas generator which generates a gas to inflate the air bag, a housing cover which contains the air bag and gas generator and an adaptor (retainer) which attaches the gas generator and housing cover. The air bag apparatus is attached to the front side of an occupant in a high-speed movable body, and, when the collision sensing device is activated by a collision, the gas is generated instantaneously from the gas generator and filled in the air bag which is folded and contained in a space formed by the surrounding gas generator, housing cover and retainer, and the housing cover is then deployed by the pressure of gas filled in the air bag which is immediately released and inflated from the opening created by the deployment toward the front side of the occupant, so that the occupant is confined to the seat and prevented from injuries by collision with the operating unit or instrument panel. In consequence, when the gas generator is activated by a collision or the like accident, the housing cover of air bag apparatus must release the bag instantaneously through its accurate deployment without spreading splinters having a danger of injuring the occupant.
As such a housing cover of air bag apparatus, JP-A-50-127336 or JP-A-55-110643 has proposed an air bag apparatus-housing cover formed from a urethane resin containing a reinforcing net mainly made of nylon.
Since a reinforcing nylon net is used, the just described air bag-housing cover is free from the problem of generating cracking on a part other than the opening portion and causing scattering of a part of the cover when the cover is deployed. However, it has other problems such as a prolonged period of time necessary for incorporating the reinforcing net or displacement of the position of the reinforcing net at the time of molding. Further, it suffers from another disadvantage of a poor productivity.
Also, JP-A-202550 proposes a housing cover having a structure wherein a surface layer made of a soft material having an A hardness according to JIS K6301 of form 30 to 70 and a core layer made of a hard resin having flexibility are integrally molded and a slit for cleavage use is arranged on the core layer. The housing cover of this proposal is desirable in terms that it has a certain degree of rigidity and gives the occupant proper feel. However, it has a problem such that, because of consisting of the core layer and surface, it should be produced by two layer molding by using an expensive double layer molding machine provided with two sets of injection mechanism. In addition, the surface layer soft material has poor surface scratch resistance and, therefore, it is needed to coat the surface after the double layer molding, thus resulting in high cost.
JP-A-38966 proposes an air bag apparatus-housing cover formed from a thermoplastic elastomer composition mainly comprising a hydrogenated styrene-conjugated diene block copolymer having an A hardness according to JIS K6301 of from 60 to 85. Although this housing cover is desirable in giving the occupant properly soft feel and realizing adaptability over a broad temperature range, it has a problem of the necessity of surface coating after injection molding because of its poor scratch resistance, thus resulting in high cost.